Effect of injection of di- and tricyclic aromatic compounds on post-combustion formation of polychlorinated dibenzo-p-dioxins and dibenzofurans

The formation of mono- to octachlorinated dibenzo-p-dioxins (PC1-8DD) and dibenzofurans (PC1-8DF) was studied using a model waste in a laboratory-scale combustion reactor with simultaneous collection of flue gas at three different temperatures (450 oC, 300 oC, and 200 oC) in the post-combustion zone. To investigate the influence of chlorination reactions and the effects of carbon backbone-containing compounds present in the flue gases, five aromatic compounds were injected into the flue gas, namely dibenzofuran (DF), biphenyl (BP), naphthalene, phenanthrene and fluorene. The injection of DF induced a reduction in the concentration of PC3-5DD, but did not significantly influence the concentration of PCDF. A reduction in the concentration of PC3-5DD was also observed during the injection of fluorene, which is structurally very similar to DF. The injection of biphenyl, naphthalene and phenanthrene had less pronounced effects on the formation of PCDD and PCDF. A possible explanation of the observed changes during injection of DF and fluorene, based on homologue profiles and affected congeners, involves formation of radical species from fluorene and/or dibenzofuran. The fluorene radical is stabilized by the delocalization of electrons across the aromatic ring structure and has the propensity to react with highly abundant hydrogen chloride, whereas the molecular species would require reaction with Cl2 or chlorine radicals. - Research highlights: → Injection of dibenzofuran (DF) and fluorene reduced the Tri-PeCDD concentrations. → Some of the reduced congeners were 2,3,7,8-substituted, i.e. TEF-congeners. → TEQ values decreased by 68-23% during DF injection, with lowest reduction at 450 oC. → A hypothetical pathway involves formation of radical species from fluorene and DF.